The subject of the present invention is a single-mode optical fiber designed to compensate for a variation in refractive index linked to thermal effects. Such an optical fiber is particularly, but not exclusively, suitable for use as a transport and amplification medium in a high-power optical fiber laser.
The term “optical fiber laser” covers any application in which the capacity of the fiber to emit light by means of electron de-excitation of elements that have previously been placed in an excited electronic state is employed. Hence this relates both to lasers and, more generally, to optical amplifiers. The optical fiber of the invention may be used in pulsed or continuous lasers.
The success of optical fiber lasers owes itself to their numerous properties: they combine high optical efficiency, a high capacity for heat dissipation, a high integration potential and excellent beam quality.
In a known manner, in an optical fiber laser, the optical fiber is used as an active medium when it is doped with rare-earth ions (erbium, ytterbium, thulium, holmium, neodymium and so on). In this case, the resonant cavity consists of the doped fiber placed between two mirrors or looped back on itself.
For power lasers, according to a first known embodiment, the structure of the fiber is composed of a single-mode core (for example made of silicon doped with rare-earth ions) and a multimode pumping cladding (for example made of doped or undoped silicon) that is larger in size than the core, thereby allowing the propagation of the various modes of a multimode pump. According to this configuration, the pump wave is guided inside the multimode cladding of the fiber while the laser wave is generated then guided solely inside the single-mode core. The laser beam thus generated is a single-mode beam that exhibits all of the desired properties of spatial and temporal coherence along with substantial power due to the multimode pumping.
Triple-clad single-mode optical fibers are also known, which allow the efficacy of amplification to be improved while retaining the single-mode character of the core. Patent FR2 974 637 thus describes an optical fiber comprising, from the center to the periphery:                a single-mode core that is at least partially doped with a rare earth;        an optically inactive intermediate cladding, the effective refractive index of which differs from the refractive index of the core by at most 1×10−3;        a multimode pumping cladding; and        an outer cladding.        
By controlling the profile of the refractive index in the intermediate cladding, it is possible to provide a small difference in refractive index between the core and the intermediate cladding while having a core with a large diameter and high doping content. The fiber thus formed therefore provides both improved amplification while retaining the single-mode character.
The term “single-mode beam” is understood to mean a beam having a divergence that is close to the minimum dictated by the diffraction of the intensity profile of the beam.
However, in all of these optical fiber structures, access to high power levels is limited due to its very low active volume and to the high level of confinement of light energy within the core, the diameter of which is between a few microns and a few tens of microns. High power densities favor the appearance of non-linear effects, which negatively affect the quality of the laser beam emitted.
In order to overcome the problems linked to non-linear effects, the most useful solution proposed in recent years has consisted in developing micro-structured fibers that allow the effective area of the core of the fiber to be increased while retaining the single-mode character of the output beam.
According to one embodiment, the micro-structured fibers take the form of a periodic or aperiodic arrangement with inclusions having a low index or high index surrounding a defect that serves as the core. Thus, it is possible to modulate the effective index of the cladding by adjusting the spacing of the array and the diameter of the inclusions. The wave is guided solely inside the core by means of a modified total reflection mechanism or by means of a photonic band gap guiding mechanism.
In order to decrease non-linear effects, it is therefore necessary to increase the area of the mode while remaining single mode. To this end, it is necessary to decrease a difference in index Δn=ncore−ncladding between the core and the cladding in order to provide the single-mode character. Control of this difference in index of the order 1×10−4 has thus been made possible by modulating the effective index of the micro-structured cladding.
However, the dramatic increase in the size of the core can only take place while also providing the most precise control possible of the difference in index Δn=ncore−ncladding between the index of the core of the fiber and the index of the intermediate cladding. Thus, in the case of wide-core or LMA (large-mode-area) fibers, the parameter Δn becomes an essential parameter for providing both the wave-guiding phenomenon and the single-mode character of the laser beam emitted at the output of the optical fiber.
However, the quantum defect, arising from the laser effect, leads to heating of the material which may, at very high power, cause a change in the refractive index of the materials and negatively affect the guiding properties of the optical fiber. Specifically, the refractive index increases with temperature. Thus, at high power, the amplification phenomenon can lead to an index increase in the cross section of the optical fiber of the order of 10−5 to several 10−4. This value may seem low when it is considered that the propagation of the wave inside a standard fiber exhibits a (core−cladding) index jump of the order of several 1×10−3. However, in the case of single-mode large-mode-area microstructured fibers, the difference in index between the core and the cladding may be smaller than 1.10−4. In these structures, the variations in index caused by heating of the material may then affect the spatial quality of the beam emitted, such as a decrease in the mode field diameter and mode instabilities.
FIG. 1A schematically shows an index profile 5 of a standard optical fiber with an index jump that is not subject to any external interference. Such an optical fiber is referred to as a “cold optical fiber,” corresponding to an inactive state of the optical fiber. For optical fibers, the index profile corresponds to the refractive index distribution of the fiber as a function of the radius of the optical fiber. In the conventional manner, the variable r (from coordinates r, θ), normalized with respect to the radius of the core, is shown on the abscissae, and the difference between the refractive index of the core and the refractive index of the fiber cladding is shown on the ordinates. The abscissa r=0 represents the center of the optical fiber. The core of the fiber, having an index ncore, extends from the center to r/rcore=±1. Δn represents the difference in index between the refractive index of the core of the fiber and that of the optical cladding. According to FIG. 1A, the index of the core of the optical fiber has a substantially constant/uniform value in the shape of a step.
According to one embodiment, it is possible not to actively dope the entire area of the fiber core. The core then consists of a central actively doped zone surrounded by a ring-shaped peripheral zone having the same refractive index as the central zone but being passively doped or undoped. It is possible to envisage other forms of dopant distribution in the core.
In FIG. 1B, 6 schematically shows the profile of a temperature gradient appearing in the core of an optical fiber when it is in an active state.
The term “active state” is understood to mean a state in which the optical fiber is currently being used to amplify a light wave. In the active state of the optical fiber, the pumping function is active and the optical fiber is subject to an increase in temperature due to thermal loading, whereas in the inactive state of the optical fiber the pumping function is not active and the optical fiber is not subject to an increase in temperature due to thermal loading. It can be seen that the temperature maximum is located in the center of the core of the optical fiber. The temperature decreases quadratically in the actively doped core with increasing distance from its center.
In FIG. 1C, 7 schematically shows an index profile of an optical fiber in an active state. It no longer takes the shape of a step. Specifically, although the difference in index at the active core/cladding interface remains unchanged, the overall profile is subject to a gradient-type progression. This modification of the index profile in the core and the cladding is caused by thermal effects that are proportional to the optical power density in the core. Typically, an index gradient of 5.10−5 can be observed on a core of 50 μm in diameter for the absorption of 75 W/m of pump power.
FIG. 2A schematically shows the index profile of an optical fiber in an inactive state and the effective indices ne01, ne11 of the two first guided modes inside the core. Only the effective index ne01 of the first mode is included between the index of the core ncore and the index of the cladding ncladding. As such, only the first mode is allowed to propagate inside the core of the fiber. Since the effective index ne11 of the second mode is not included between the index of the core and that of the cladding, the second mode is therefore not allowed to propagate inside the core. The effective index may be likened to the refractive index from the point of view of the light propagating along one mode inside the structure of the core.
In FIG. 2A, 8 shows the near-field intensity distribution of the fundamental mode as output from the optical fiber. It is observed that the beam emitted as output from such an optical fiber is single mode.
In FIG. 2B, the index profile of an optical fiber in an active state is shown. The effective index of the second mode ne11 is now included between the index of the core and the index of the cladding and is thus confined within the core. Thus, the second mode can be propagated inside the core. In FIG. 2B, 9 shows its near-field intensity distribution as output from the optical fiber. It is observed that the beam emitted as output is multimode, thereby confirming the negative effect on the spectral quality of the beam due to thermal effects.
Thus, even though the new microstructured optical fiber architectures allow the thresholds at which non-linear effects occur to be pushed back, there is still currently no technical solution to the problem of controlling the index profile of the optical fiber, due to the thermal effects that appear in fibers operating at substantial power ranges.